1. Field of the Invention
The present invention relates to an uninflammable insulating liquid and a stationary induction apparatus, e.g., a voltage transformer or a transformer using an uninflammable insulating liquid as a coolant.
2. Description of the Prior Art
A mineral oil-based insulating oil conventionally used widely as an insulating and cooling medium of an oil-sealed stationary induction apparatus is flammable. A strong demand has arisen for such an oil-sealed stationary induction apparatus to use an uninflammable insulating liquid in view of prevention against disasters. Polyclorinated biphenyl (PCB), which was put into practice for the first time as an uninflammable insulating liquid to replace the mineral oil-based insulating oil, was totally banned because of its accumulated toxicity. Hence, studies and developments have been made so far at various laboratories to develop a pollution-free uninflammable insulating liquid.
Uninflammable insulating liquids that can be currently actually used as the pollution-free insulating liquid and coolant are roughly classified into the fluorocarbon-, chloride-, ester-, and silicone oil-based uninflammable insulating liquids. An example of the fluorocarbon-based uninflammable insulating liquid includes, e.g., perfluorooctane (CSF.sub.8 F.sub.18), perfluorocyclicether(C.sub.8 F.sub.16 O), and perfluoropolyether. The fluorocarbon-based uninflammable insulating liquid is a completely uninflammable liquid which is chemically very stable and does not have a flash point or fire point. The chemical formula of perfluoropolyether is: ##STR1## wherein m and n take various values to provide a multiple of types of perfluoropolyether having different boiling points and viscosities.
An example of the chloride-based uninflammable insulating liquid includes, e.g., a mixture (Japanese Patent Laid-Open No. 63-216206) obtained by mixing phosphate-based. tricresyl phosphate {(CH.sub.3 C.sub.6 H.sub.4 O)3.PO} and perchloroethylene (Cl.sub.2 C : CCl.sub.2) and a mixture (Japanese Patent Laid-Open No. 59-20909) obtained by mixing perchloroethylene and Freon. Although these chloride-based uninflammable insulating liquids were developed as products having no toxicity, as the great deal of attention has begun to be paid on environcmental issues, it became difficult to put them into practical use. More specifically, limitation has begun to be put on products that can lead to destruction of the ozone layer, as is seen with Freon. Therefore, all the chloride-based products tend to be avoided.
As an ester-based uninflammable insulating liquid, e.g., polyol ester {C(CH.sub.2).sub.4 --(COOR).sub.4 where R is an alkyl group) is commercially available as Midel-7131 (tradename) manufactured by Beck Blektroisolier-System Co. and sold by DAINICHISEIKA COLOUR & CHEMICALS MFG. CO., LTD. As a silicone oil-based uninflammable insulating liquid, e.g., polydimethylsiloxane is commercially available. The ester- and silicone oil-based uninflammable insulating liquids are partly put into practical use as, e.g., a vehicle transformer since they do not pose pollution or an environmental problem. However, these liquids are said to be fire retardant and not completely uninflammable. More specifically, when compared to the mineral oil-based insulating oil, they merely have a very high flash point of several hundreds of .degree.C. and do not catch fire easily. To have a flash point is a drawback, and development of a completely uninflammable liquid practically having no flash point is demanded. The fluorocarbon liquid described above is higly evaluated in terms of complete uninflammability.
The fluorocarbon liquid described above, however, has a large specific gravity and is very expensive.
The fluorocarbon-based liquid is partly put into practical use as it is completely uninflammable, as described above, and is chemically inert. However, its specific gravity is twice that of the mineral oil-based insulating oil, and an electric instrument filled with the fluorocarbon liquid becomes very heavy. In addition, the cost of the fluorocarbon liquid per unit volume is higher than that of the mineral oil-based insulating oil by 100 times, resulting in an increase in weight of the overall electric instrument and cost. The fluorocarbon liquid is chemically inert. Accordingly, it can dissolve only a Freon-based material and a fluorocarbon-based material which is identical to itself. Hence, it is difficult to reduce the weight and cost by dissolving and mixing those materials in the fluorocarbon liquid.